The invention relates to cranes and hoists especially for marine use; and also to hauling by ropes such as in mooring or towing activities where the load on the winding drum is liable to variation which, if not compensated for, may cause unacceptable forces on equipment.
It has been proposed in U.S. Pat. No. 1,999,936 to drive a winding drum by a main motor through an epicyclic gear mechanism and by an auxiliary motor through the same mechanism. The auxiliary motor is permanently coupled to a messenger drum on which a messenger rope is wound having one end attached to an airplane required to be hoisted from the sea onto the deck of a ship by a hoist rope wound on the winding drum.
A pawl-and-ratchet is arranged when engaged to prevent rendering rotation of the messenger drum.
A manually-operable reversing switch and a rotation-sensitive on/off switch control the engagement of the pawl. The on/off switch is such that it closes and remains closed so long as the messenger drum is "heaving" i.e. winding in rope. Only change-over of the manual switch can change the condition of the pawl and is effective only when the on/off switch closes.
The pawl and ratchet is arranged such that the auxiliary motor can turn the winding drum in the "heave" sense regardless of the condition of the pawl.
However, the pawl must be engaged to provide torque reaction through the gear mechanism to enable the main motor to drive the winding drum in the "heave" sense.
The messenger rope system allows the hoist rope and hook to be lowered towards the airplane with a motion superimposed on the hook which partly corresponds to the up-and-down motion of the airplane caused by the sea-state.
In an alternative version described in U.S. Pat. No. 1,999,936, the messenger rope is dispensed with and the auxiliary motor is operable to eliminate slack from the hoist rope attached to the airplane.
Cranes hoists and analogous equipment have considerable inertia in their gear mechanisms and therefore the proposal in U.S. Pat. No. 1,999,936 has a serious drawback because the epicyclic gear mechanism in practice would have considerable inertia.
Another drawback arises from the need to use a complicated electrical system to operate the pawl even in the simple case of slack rope elimination (i.e. where no messenger rope is used).
Where the messenger rope is used in order to give compensation for sea-state motion before hoisting of the load is initiated, the epicyclic gear mechanism is incapable of allowing the hook motion to correspond exactly with sea-state motion. At least the amplitude of hook motion must of necessity be different from the amplitude of the wave induced motion of the airplane.
Therefore, attachment of the hook to the airplane is very difficult in mild sea-states and impossible in severe sea-states. Airplanes are hoisted aboard only where sea-states are reasonable.
It can be said definitely that such a system is entirely inapplicable to the attachment and hoisting of loads under difficult sea-states typically encountered in deepwater oil exploration and drilling operations where wave heights commonly exceed 4 to 6 feet but loads must nevertheless be hoisted.